1. Field of the Invention
The present invention relates to a method for forming homogenous density borophosphosilicate glass layer, and particular relates to a method for controlling ratio of component elements of borophosphosilicate glass layer by controlling delivered timing of reactants.
2. Description of the Prior Art
In semiconductor fabrication process, silicon dioxide is a broadly used dielectric material, no matter is used to be passivation layer, barrier layer, capacitor or other purpose. Further, except directly use thermal oxidation to form silicon dioxide layer on a silicon substrate, silicon dioxide usually is formed by deposition method.
Reactants of silicon dioxide of conventional deposition method usually are silane or tetraethyl-orthosilicate (TEOS). Because step coverage of silicon dioxide formed by silane is worse and both wafer and chamber is easy to be polluted during reacting process of silane, and because step coverage of silicon dioxide formed by tetraethyl-orthosilicate is batter, silane is gradationally replaced by tetraethyl-orthosilicate.
Besides, owing to critical dimensional of semiconductor device is continuously shrunk, deposited silicon dioxide layer usual faces some disadvantages such as gap fill is bad or surface is not smooth, and then it is necessary to perform thermal flow process after depositing process is finished to let all gaps are filled and surface is more smooth. However, because glass translation temperature of silicon dioxide formed by tetraethyl-orthosilicate usually is larger than 1000.degree. C., thermal flow process usually come in with thermal diffusion that will degrade quality of semiconductor device. Thus, to reduce the glass translation temperature, one effective way is doping impurities, such as boron or phosphorous, into silicon dioxide to reduce the glass translation temperature, and the most popular way is doping both boron and phosphorous into silicide dioxide at the same, which is named as borophosphosilicate glass. Owing to glass translation temperature of borophosphosilicate glass is about from 850.degree. C. to 950.degree. C. and mechanical stress of borophosphosilicate glass is less than mechanical stress of pure silicon dioxide, borophosphosilicate gradationally become most popular material for metallization process.
Further, because step coverage of silane is bad than tetraethyl-orthosilicate and reacting process of silane requires virulent reactants such as B.sub.2 F.sub.6 and PH.sub.3, forming process of borophosphosilicate usually uses organic compounds, such as tri-ethyl-borate (TEB) and tri-ethyl-phosphate (TEPO), as reactants, and use ozone to reduce required reacting temperature during reacting process.
Because organic compounds, such as TEOS, TEB and TEOS, usually are liquid in room temperature, organic compounds can not be directly delivered into chamber. As usual, popular methods at least includes heats container of liquid organic compound to increase saturated vapor pressure and delivers carrier gas into the container to increase gaseous pressure of liquid organic compound. Herein, flow of the gaseous organic compound can be controlled by adjusting temperature of container, adjusting flow of carrier gas (by application of mass flow meter) or adjusting flow of liquid organic compound (by application of liquid flow meter) and so on. Obviously, because property of borophosphosilicate is strongly affected by ratio of boron and phosphor, how to exactly control flow of organic compounds is a master key of borophosphosilicate fabrication.
Further, because these organic compounds are delivered into chamber after required reacting environment has been established, flow of these organic compounds should be instantly changed from zero to required account on paper. However, owing to limitation of practical ability of machines. Real relation between flow and time usually is not stable at incipient period and is stable after a buffer period, as shown in FIG. 1A and FIG. 1B. In this way, before borophosphosilicate is formed and even in incipient period of depositing process, ratio between all used organic compounds is not fixed and following byeffects are unavoidable: first, density of boron/phosphor is different to default density and then glass translation temperature of bottom part of borophosphosilicate glass layer is different to glass translation temperature of bulk part of borophosphosilicate glass layer; second, account of TEB and/or TEPO is too much to be totally used, and then part of TEB and/or TEPO exists between substrate and bottom part of borophosphosilicate glass layer as liquid residual.
Significantly, because result of thermal flow process strongly depends on glass translation temperature, whenever glass translation temperature of bottom part of borophosphosilicate glass layer is higher than glass translation temperature of bulk part of borophosphosilicate glass layer, as FIG. 1C shows, borophosphosilicate glass layer 12 can not totally fills gaps between semiconductor structures 11 on substrate 10, and then voids 13 is appeared. Moreover, as FIG. 1D shows, because sequential processes after borophosphosilicate 12 is formed at least includes etching process and cleaning process, whenever etching rate of bottom part of borophosphosilicate glass layer 12 is higher than etching rate of bulk part of borophosphosilicate glass layer 12, undercuts 14 will be happen. Obviously, if neighboring undercuts is too close, not only structural stability of borophosphosilicate glass layer 12 is degraded but also neighboring undercuts 14 may be short whenever metal 15 is filled into these undercuts 14.
Furthermore, as FIG. 1E shows, whenever liquid organic compounds 16, TEB and/or TEPO, remains between substrate 10 and bottom part of borophosphosilicate glass layer 12, adhesion between substrate 10 and bottom part of borophosphosilicate glass layer 12 will be degraded, and than borophosphosilicate glass layer 12 may faces following disadvantages: peeling and lifting, as FIG. 1F shows.
According to previous discussion, current fabrication process of borophosphosilicate glass layer can not effectively prevent non-homogenous density of boron and/or phosphor, and then some bad byeffects, such as void, undercut and degraded adhesion, are unavoidable. In this way, in order to fully produce result of good step coverage and low glass translation temperature of borophosphosilicate glass, it is desired to develop a method for forming homogenous density Borophosphosilicate glass layer.